The third year of junior high school physical pulley group, pressure formula

w has = mgh mass of the weight, lifting height.

Total w = fs distance between the pulling force and the pulling force.

n=w has w total.

It's the same with horizontal placement.

Liquid to container pressure p = p liquid gh liquid density idiom g multiplied by height.

The pressure of the object on the table, p=f s, the pressure divided by the contact area, you have to write the situation clearly. Such a general writing must be the same.

It's just that w is sometimes multiplied by the distance w has = mgsw is always equal to the pulling force f multiplied by the corresponding distance.

There's an extra friction when you're placed horizontally, so there's always a little more w.

The pressure of the liquid on the container (bottom) p=p liquid gh , the pressure is equal to the weight of a cylindrical body with the bottom area of the container as the bottom area and the height of the water surface as the base.

The pressure of the object on the table is equal to the gravitational force of the object, and then according to p=f s (the slash cannot be played).

Objects transfer pressure, liquid transfer pressure, this is what our teacher taught, hehe, understand it yourself, so that the memory is deep, and then do a few different types of problems.

Then your physics will be topped with melons

Good luck and ask me any questions.

P.S. We learned this in the second year of junior high school ( Lalala, I'm still the representative of the physics class.

Physical horizontal (transverse) pulley block.

The number of rope segments on the movable pulley is n

The tension at the free end of the rope is the brigade f, and the object resistance is f

The distance of the free end of the rope is s, the distance of the object is s1, the speed of the free end of the rope is v, and the speed of the object is v1

The relationship between the forces:

f=f n distance relation: s=ns1

Velocity relation family: v=nv1

Pulley block is a very important concept in junior high school physics learning, using pulley block to change the direction of force and make objects move. The formula regarding the block of pulleys is mainly related to the action of force, the acceleration of the object, the gravitational force, and other factors.

There are three factors that need to be taken into account in the formula for a pulley block: force, acceleration, and gravity. First of all, the magnitude of the force is related to the force exerted by the pulley block, through which the direction of the force can be changed.

In practice, the magnitude of the force can be calculated by applying different amounts of force. Secondly, the acceleration of the pulley block is also something to consider, and the force of different magnitudes applied by the pulley block will affect the acceleration of the object. Finally, the gravity of the block of pulleys is also something to consider, as the force exerted by the block of pulleys interacts with gravity, so the magnitude of the gravitational force needs to be calculated.

In junior high school physics, the general form of the pulley block formula is f=ma, in which f represents the force, m represents the mass of the object, and a represents the acceleration and degree of the object. When there is only one pulley in the pulley block, the teaching assistant makes the gravitational upward force fg and the downward force f equal, i.e., fg=f, and the pulley block is equivalent to a fixed pulley, a=0. When there are multiple pulleys in a pulley block, the force and acceleration can be calculated according to the number of pulleys and the arrangement of the pulley block.

In addition, the friction of the pulley also needs to be considered in the pulley, because the friction affects the movement of the pulley, and the limb sheds fluid makes the formula of the pulley block more complicated. In practice, it is necessary to consider the type of pulley, the material of the pulley, the diameter of the pulley and other factors, so as to calculate the appropriate pulley group formula.

In short, the formula of the junior high school physics pulley block is based on f=ma as the core, and is calculated according to the force, acceleration and gravity. In practical application, it is necessary to pay attention to the influence of pulley friction, and select appropriate formulas for different pulley sets to obtain correct calculation results. <>

Knowledge point 1: The concept of useful work, extra work, and total work. (1) Useful work:

The work that must be done to be useful to people can be understood as a kind of "purpose" work. (2) Extra work: work that is useless to people but has to be done extra.

For example, the work done to overcome the friction between the weight of the machine and its own parts. (3) Total work: It is the work done by people when they use machinery.

Including the useful work done by mechanically overcoming the resistance of the object and the additional work done by overcoming the gravity and friction of the machine itself, there is w total w and w amount.

Knowledge point 2: mechanical efficiency. The higher the mechanical efficiency, the greater the ratio of useful work to total work.

It has nothing to do with whether the machine is labor-saving, how much work the machine does, and the size of the power. Mechanical efficiency is a ratio, has no units, and is usually expressed as a percentage. The mechanical efficiency is always less than 1 when the work is done with machinery.

Note that mechanical efficiency and power are two unrelated concepts. For a certain machine, power indicates the speed of work done, and mechanical efficiency indicates the ratio of useful work to total work when machinery does work, that is, the physical quantity of the degree of energy utilization. For high-power machinery, the mechanical efficiency is not necessarily high; Machinery with high mechanical efficiency does not necessarily have high power.

Knowledge point 3: Calculation of mechanical efficiency when several common machinery is doing work. (Noted).

The useful work w in Figure (1) is gh; Total fs; η＝gh/fs=g/nf

Figure (2).

w has (g f float) h

w total fs (g f float) h fs = (g f float) nf

Figure (3).

w has f mos thing.

Total FS rope.

f Mo s matter fs rope = f Mo nf Figure (4) w has gh; Total fs; η＝gh/fs

Knowledge point 4: Mechanical efficiency experiment of pulley block. (1) Principle:

W has w total gh fs. (2) Measuring tools: scale, spring dynamometer.

3) Precautions: The spring dynamometer must be pulled at a constant speed to raise the hook code to ensure that the size of the dynamometer remains unchanged. (4) Research Methods:

Control variable method. (5) Conclusion: 1. Factors affecting mechanical efficiency:

The weight of the lifting weight, the weight of the movable pulley, and the friction (note: the winding method and the lifting height of the heavy object do not affect the mechanical efficiency of the pulley block); 2. Specific results: the heavier the weight lifted by the same pulley, the higher the mechanical efficiency, and the heavier the moving pulley, the greater the friction and the lower the mechanical efficiency when different pulleys lift the same weight.

Knowledge point 5: Methods to improve mechanical efficiency. The mechanical efficiency of a mechanical device is not fixed, and it can be improved by taking the following measures:

1) Keep w unchanged and reduce w amount, such as: reduce the weight of the machinery or add lubricating oil to reduce friction; (2) Keep the amount of w unchanged and increase w there, the heavier the weight lifted by the same pulley, the higher the mechanical efficiency.

Leverage equilibrium condition: f1l1*f2l2

Buoyancy vs. pressure:

p = f s (solid pressure).

f=ps= gh*s (according to the above formula).

p= gh (liquid pressure).

f= liquid v drain g (buoyancy).

The atmospheric pressure can support 760mm of mercury.

g is generally taken, if it is marked after the question (g=10n kg), then use this hint to calculate.

Pulley set: Master 5 letters:

s (distance pulled),

n (number of ropes connected to movable pulleys),

h (elevated height),

g (gravity),

f(tensile force) s=n*h

f=1 n*(g object + g motion).

The gravitational force of the moving pulley is sometimes overlooked.

I'm in junior high school, so I don't know if I can help you.

These formulas are available in physics textbooks, so you can check them out for yourself.

You'd better learn the language first.

Human-to-ground pressure: F = 4000 Pa x 250C = 4000 N x 250 10000 = 100 N

Man's own gravity: g man = m man xg = 50kg = 490n

Judging from the above, people are subjected to an upward pull: f person = g person - f land = 490n - 100n = 390n

The principle of saving half of the effort by moving pulley: G object + G motion = 2xf person, that is, G object = 2xf person - G motion = 2 x390N- 50N = 730N

The object is submerged in water (the movable pulley does not enter the water), as follows:

Matter = 3000m kg? Mistaken! Let me first assume that the correct thing is object = 3000kg m 3 (kg cubic meter), object volume: v object = g object (g x object).

The buoyancy force experienced by the object: f float = water xg xv object = water xg xg object (g x matter) = water xg matter.

f float = 1000kg m3 x730n (3000kg m 3) =

The tensile force f experienced by the object'Pull = g matter -f float = 730n =

The pull f'Person = (f'Pull+g-move) 2 =( =

The pressure of the person on the ground f'地 =g人-f'People = 490n

The pressure of the person on the ground p'=f'Ground s contact = =

I was wrong, it was the pulley block that was not a movable pulley.

(1) F = (G Matter + G Wheel) 2 = (500 + 80) 2 = 290 (N) (2) Mechanical Efficiency = Useful Work Total Work = GH FS = 500 * H (290 * 2H) =

The easiest way to do it is easy to learn and learn.

2 refers to the number of rope segments to lift the object, and h refers to the lifting height.

Solution: (1) f = 1 2 (g object + g wheel) = 1 2 (500 + 8x10) = 290n

2): W total = FS = 290 x 2 x 5 = 2900 (J) W useful = GH = 500 x 5 = 2500 (J).

w useful w total = 2500 2900 =

The weight of half of the box + the weight of the movable pulley 1050N

Mechanical efficiency of pulleys: (ignoring rope weight and friction).

When used to lift heavy loads: =w has w total.

g-object is the number of segments of rope that bear the weight of the object).

G object G thing + G motion (G motion is the weight of the movable pulley).

GH FS when used to move objects horizontally (i.e. to overcome friction):

Ropes. f is the frictional force experienced by the object, and the s object and the s rope are the distance traveled by the object and the rope respectively).

A, fixed pulley, f1=f

B, movable pulley, f2=f2

C, movable pulley, f3 = 2f (the difference between C and B is that the position of the tension is different, in fact, such problems can be solved by using the balance of force).

If the objects are moving at a uniform speed on the level ground, and the friction force f is the same, D is selected